A liquid crystal panel included in a liquid crystal display device, such as a liquid crystal television set, does not emit light, and thus a backlight unit is required as a separate lighting device. As the backlight unit, a backlight unit installed behind the liquid crystal panel (on a side opposite from a display surface side) is well known. The backlight unit includes a chassis having an opening in the surface on the liquid crystal panel side, a number of light sources (such as cold cathode tubes) housed in the chassis as lamps, an optical member (such as a diffuser plate) arranged at the opening of the chassis to efficiently discharge light emitted from the light sources to the liquid crystal panel side, and a reflection sheet laid in the chassis to reflect the light emitted from the light sources to the optical member side and the liquid crystal panel side.
In such a backlight unit, when the light sources are configured to emit linear light, the optical member converts the linear light into planar light, thereby achieving the uniformity of illumination light. However, if the conversion into planar light is insufficient, a banded lamp image is generated along the arrangement of the light sources, which deteriorates the display quality of the liquid crystal display device.
To achieve the uniformity of the illumination light from the backlight unit, it is desirable to reduce the distance between the adjacent light sources by increasing the number of light sources to be arranged, or to increase the diffusing power of the diffuser plate, for example. However, the increase in the number of light sources leads to an increase in cost of the backlight unit and an increase in power consumption. Furthermore, the increase in the diffusion power of the diffuser plate makes it difficult to increase the brightness, resulting in a problem of the necessity to increase the number of light sources. Under such circumstances, there is known a backlight unit that maintains the brightness uniformity while suppressing power consumption as disclosed in Patent Document 1 described below.
The backlight unit disclosed in Patent Document 1 has a configuration in which a diffuser plate is arranged on a light exit side of a plurality of light sources, and a dimming dot pattern having a total light transmittance (aperture ratio) of 62 to 71% and a haze of 90 to 99% is printed on the diffuser plate. Particularly, in the configuration, the dot diameter immediately above the light sources is large and the dot diameter decreases with distance from the light sources. According to this configuration, the light emitted from the light sources is efficiently utilized, thereby obtaining a sufficient brightness value without increasing the power consumption of the light sources and enabling irradiation of light having uniform brightness.
Patent Document 1: Japanese Unexamined Patent Publication No. 2005-117023